In the old binary export format, parameter names for parameter lists
which contained only types where never written, so this problem didn't
come up.

Fixes #25101.

Change-Id: Ia8b817f7f467570b05f88d584e86b6ef4acdccc6
Reviewed-on: https://go-review.googlesource.com/116376Reviewed-by: Matthew Dempsky <mdempsky@google.com>